1. Field of the Invention
This invention relates generally to electro-optical reconnaissance systems whose angular resolution is greater than the product of the exposure time and the angular rate of image motion. The invention is a forward motion compensation (FMC) system that permits full resolution performance when the target line-of-sight-angular-rate-exposure-time product is greater than the angular resolution of the system. The system includes optics, a mechanical shutter and full frame CCD.
2. Related Art
Aerial reconnaissance systems have undergone a dramatic transition in the past two decades with the replacement of photographic film by electro-optic image sensors. With the advent of wafer-scale focal planes that provide sufficient coverage and resolution, reconnaissance systems are being designed to utilize electro-optic sensors configured as large format area arrays. These electro-optic (xe2x80x9cEOxe2x80x9d) reconnaissance imaging systems most often employ charge-coupled devices (xe2x80x9cCCDsxe2x80x9d) operating in the visible and near-infrared regions of the electromagnetic spectrum to capture the image of the target or scene. The ability to operate in a real-time environment and in low ambient light conditions are just a few of the reasons why electro-optical-based reconnaissance imaging systems are increasingly replacing film-based reconnaissance systems.
One of the more frequently encountered problems in designing aerial reconnaissance imaging systems is determining the most effective method of compensating for image smear or blurring. Typically, smearing occurs when low ambient light conditions prevent an imaging system from using sufficiently short exposure times, resulting in a blurred image due to the forward motion of the aircraft. In other words, smearing occurs as a result of the relative motion between a scene or target to be imaged and the imaging system. Therefore, in order to prevent the degradation of the information contained in a recorded image, an ideal reconnaissance imaging system must utilize some means of image motion compensation (xe2x80x9cIMCxe2x80x9d) for image smear.
Different reconnaissance mission operating scenarios can present different image motions that should be compensated for. The goal of any image motion compensation system, of which a forward motion compensation (xe2x80x9cFMCxe2x80x9d) system is a specific category, is to reduce the image smear that occurs when the target line-of-sight-angular velocity is significantly different from the camera angular velocity.
Early reconnaissance systems comprised linear arrays that operated at high altitudes, thereby minimizing the angular motion effects proportional to the aircraft velocity/altitude ratio. However, when low flying mission scenarios are required to avoid detection of the reconnaissance aircraft, forward motion compensation is necessary to maintain image resolution. Several conventional methods of IMC have been developed to meet these image resolution requirements.
For example, U.S. Pat. No. 4,505,559, issued Mar. 19, 1985 to Prinz, discloses an approach wherein an instantaneous line-of-sight controls the motion of the film used to record the image. U.S. Pat. No. 4,157,218, issued Jun. 5, 1979 to Gordon et al., also uses a film drive to compensate for the forward motion of the image. Mechanical means are used in U.S. Pat. No. 4,908,705, issued Mar. 13, 1990 to Wight, where the imaging array physically moves to reduce the smear.
U.S. Pat. No. 5,155,597 to Lareau et al., issued Oct. 13, 1992, discloses an equation that described the correction for the image motion in the side oblique scenario by transferring the charge in a column segmented CCD array at different transfer rates corresponding to the depression angle.
However, these aforementioned image motion compensation techniques are inadequate to provide for image motion compensation in each of the various mission scenarios described above. What is needed is an electro-optical reconnaissance system that provides adequate image motion compensation in forward oblique, side oblique, and vertical orientations. In addition, it is desirable that this reconnaissance system be low cost.
The present invention provides a system and method for the compensation of image motion during reconnaissance missions. According to a first embodiment of the present invention, the electro-optical reconnaissance system includes an imaging focal plane array (FPA), such as a charge-coupled device (CCD), to record a target scene. The focal plane array includes a main format area having a plurality of photo-sensitive cells arranged in rows and columns. The reconnaissance system also includes a shutter having a window (or exposure slit) that moves across the imaging device. In order to compensate for the forward motion of the vehicle, such as an aircraft, the charge in the imaging device is transferred across the device. The rate of charge transfer is uniform across the focal plane array, but varies in time in accordance with the portion of the target being imaged, where the portion of the target scene being imaged is defined by the position and width of the shutter slit. The charge transfer rate is varied based on the position of the shutter slit over the imaging device. A camera control electronics unit controls the position of the shutter slit and processes target scene information, light levels, and reconnaissance mission requirements in order to determine to the rate of motion of objects contained in the portion of the target scene viewed by the focal plane array. As a result, the camera control electronics unit can generate an appropriate clocking signal to perform forward motion compensation (FMC) in a variety of target viewing modes, including forward oblique, side oblique, and vertical modes of operation.
According to a second embodiment of the present invention, a method for providing forward motion compensation for the electro-optical reconnaissance system is utilized in the camera control electronics unit. First, a light sensor measures the light level of the scene to be imaged by the reconnaissance system. Next, the measured light level is compared to a predetermined light level value. For example, the predetermined light level value can correspond to a given solar angle above the horizon. If the measured light value is greater than the standard value, an exposure time is determined by comparing the measured light level to a primary exposure time look-up table. If the measured light value is less than the standard value, the exposure time is determined by comparing the measured light level to a low exposure time look-up table. By determining the proper exposure time, the proper shutter slit width and shutter slit speed are determined. Next, a forward motion compensation profile is determined corresponding to the exposure time and mission parameter inputs. For example, the mission parameters can include aircraft velocity, altitude, and camera look angle. This FMC profile corresponds to the clocking signal that is used to drive the focal plane array of the reconnaissance system in order to perform FMC.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.